activate.sh: drop $_OLD_VIRTUAL_PATH, use remove_from_path instead

[blueyed]

Using $_OLD_VIRTUAL_PATH will overwrite any changes to $PATH inbetween
activating and deactivating the virtualenv. It also does not behave
well with exec $SHELL (reloading your shell).

remove_from_path is taken from pyenv/rbenv's pyenv-which/rbenv-which
shim (https://github.com/sstephenson/rbenv/blob/e851250/libexec/rbenv-which#L18-L27).

[Ivoz]

@blueyed I like the idea of this, however concerned that this might not be portable shell code? We don't have the luxury of demanding bash.

[BrownTruck]

Hello!

As part of an effort to ease the contribution process and adopt a more standard workflow pip has switched to doing development on the master branch. However, this Pull Request was made against the develop branch so it will need to be resubmitted against master. Unfortunately, this pull request does not cleanly merge against the current master branch.

If you do nothing, this Pull Request will be automatically closed by @BrownTruck since it cannot be merged.

If this pull request is still valid, please rebase it against master (or merge master into it) and resubmit it against the master branch, closing and referencing the original Pull Request.

If you choose to rebase/merge and resubmit this Pull Request, here is an example message that you can copy and paste:

Using $_OLD_VIRTUAL_PATH will overwrite any changes to $PATH inbetween
activating and deactivating the virtualenv.  It also does not behave
well with exec $SHELL (reloading your shell).

remove_from_path is taken from pyenv/rbenv's pyenv-which/rbenv-which
shim (https://github.com/sstephenson/rbenv/blob/e851250/libexec/rbenv-which#L18-L27).

---

*This was migrated from pypa/virtualenv#722 to reparent it to the ``master`` branch. Please see original pull request for any previous discussion.*

[blueyed]

@BrownTruck / @dstufft
Before rebasing/resubmitting this I'd like to have some feedback about if this is feasible / useful, and would be considered to be merged after all.

@Ivoz
I could investigate to make this POSIX compatible.

[BrownTruck]

This Pull Request was closed because it cannot be automatically reparented to the master branch and it appears to have bit rotted.

Please feel free to re-open it or re-submit it if it is still valid and you have rebased it onto master or merged master into it.